The present invention relates generally to methods of processing starch which include steps of adding water to starch to form starch slurries. More specifically the invention relates to improvements in the mechanical dewatering of starch slurries. During the manufacture of modified starches derived from various natural sources such as corn, wheat, rice, tapioca, and the like various processes are carried out to remove the starch in its native form from the plant seed or tuber.
The raw material for most starch production in the United States is dent corn where the starch is located in the floury endosperm as large round granules associated with a relatively small amount of protein. The starch granules from corn are removed during a process commonly referred to as wet milling. (A comprehensive description of corn wet-miffing can be found in Principles of Cereal Science and Technology, R. Carl Hoseney, pages 153-156, 1986.) During this process, the corn kernels are subjected to both mechanical and chemical forces that separate the various components found both on the exterior of and within the kernel. Shelled grain is prepared for milling in cleaners where air currents blow light chaff, and screens remove heavier pieces of foreign material. The cleaned corn is softened by soaking (steeping) for 30 to 50 hours in warm (50.degree. C.) water acidified with sulfur dioxide to a pH of 3 to 4. The dilute sulfurous acid aids in the disintegration of the protein matrix so that starch and other components can be separated more easily. During steeping, the water content of the grain increases to 45%. The softened kernels are separated from the steepwater and then coarsely ground with water in degerminating or attrition mills. The lighter germ floats to the surface where it is removed from the other particles in flotation troughs or by hydroclone separators in which germ passes out in the overflow and the remaining mixture of starch, gluten and hulls pass out in the underflow.
The starch containing mixture is ground in a mill to separate the starch and gluten from the hulls. The starch and gluten are then separated from each other by means such as centrifugation. After separation from the gluten the starch is filtered by means such as screening and is typically washed or passed through a hydroclone. The starch that has been removed is washed and screened to remove undesirable material from reaching the next stage of manufacture. This aqueous starch slurry is then introduced into several process streams where various chemical modifications take place, causing many changes to the physical properties of the resulting granules. At this point the starch slurry is typically subjected to drying by a combination of mechanical and thermal steps.
Drying the suspended starch slurry after chemical modification involves reducing the approximately 20 gallons of water per 100 pounds of corn that has been added during the wet-milling process. This water must be removed before the product is packaged and marketed. The present invention relates to methods which help facilitate the removal of residual water from the starch slurry, thus allowing the manufacturing process to operate in a more efficient manner and allowing the starch product to be further dried, packaged and shipped for use in an industrial setting. The starch slurry is first mechanically dried in a dewatering step using means such as a plate or frame filter, centrifuge, drum dryer or most preferably a vacuum drum filter. The mechanical dewatering step typically produces a starch cake having a moisture content in the range of from 40 to 50 percent by weight. This leaves essentially two components remaining, a starch wet-cake and the water that was removed from the slurry, commonly referred to as filtrate. The wet-cake moisture content is the critical measurement regarding the efficacy of the dewatering step. The starch cake is then thermally dried, for example, in a tunnel or flash dryer.
It is desired to reduce the quantity of moisture present in the starch to promote safety, ease of handling and transfer as well as to reduce potential microbiological growth. In addition it is generally desired that as much water as possible be removed during the mechanical dewatering step in order to minimize energy consumption and maximize productivity (throughput) during subsequent thermal drying. Because of the large capital investment represented by thermal dryers for starch processing numerous efforts have been made to increase their throughput. Such efforts to improve the throughput of starch thermal dryers have been limited, however, by the tendency of drying starch to partly gelatinize when subjected to intense heating conditions. As dryer temperature and speeds are increased to gain output, more of this partial gelatinization occurs. In the art, the resulting dry starch is referred to as "horny" because of the very hard surface that is thus obtained. This partially gelatinized starch does not readily reslurry when the starch is used. A partially gelatinize starch becomes viscous and difficult to reslurry, with resulting difficulty in increasing solids content during processing. Moreover, when cooked, the partially gelatinized starch does not cook out properly and end use properties such as strength in a starch coated paper are adversely affected. These problems of partial gelatinization are even more significant when the starch in question is a substituted starch because such starches have an even greater tendency than normal to gelatinize upon heating. There is thus a desire in the art for methods to improve starch dryer throughput while avoiding partial gelatinization of starch and providing improvements in the quality of resulting starch products.
Surface active agents, or "surfactants" are widely used in industrial processes to decrease the surface tension of water and aqueous dispersions. Surfactants are generally divided into three classes: emulsifiers, wetting agents, and detergents. All surfactant types are similar in that they have their effect at the surfaces of materials in solution. Emulsifiers keep aqueous, immiscible compounds in suspension by coating the suspended phase and decreasing surface tension of the dispersing phase at the interface. Detergents are emulsifiers that affect water/oil interfaces to bring oily, solid materials into solution. Wetting agents allow the dispersion of water onto or into other materials by decreasing surface tension, thus increasing flowability. While surfactants are generally used to maintain the physical integrity of dispersions, they can also be used to create differences between dispersion components. A common system for using surfactants to separate dispersion components is industrial waste water purification, notably waste sludge water removal.
Surfactants used in waste sludge water removal are usually oil based and act as emulsifiers in that the surfactant surrounds the solid particle with an oily film which repels water and thus promotes the separation of sludge from the treated water. This method results in an oily residue on the solid particle after the water has been removed. In the case of processing waste sludge, this presence of surfactant residue on the filtered solids is preferred because it is the sludge which is disposed of thus minimizing the presence of surfactant residue in the separated waste water which is typically subjected to later purification steps. Nevertheless, the use of oil based surfactants such as are used in waste sludge processing can present problems in continuously operating aqueous systems where water is the primary medium and very little oily material is present. If the surfactant is not miscible in the water phase, problems in processing equipment can occur and have detrimental effects on the resulting product. Processing problems are compounded if the product is processed requiring elevated dosages of surfactant to be effective.
Surfactants have been used in starch processing on a limited basis. Specifically, surfactants have been used at low concentrations, i.e. less than 200 ppm as antifoams in starch processing. In addition, U.S. Pat. No. 5,536,326 discloses the use of a particular surfactant composition comprising polybutene and at least one surfactant having a melting point less than 20.degree. C. which enhances the removal of water from starch when used at concentrations of from about 100 to 500 parts per million (ppm). Nevertheless, there remains a desire in the art for improved methods and compositions to enhance dewatering of starch. In particular, there remains a desire in the art for dewatering compositions and methods wherein the dewatered starch remains substantially free of the surfactant and other compositions used in the dewatering process.